Genetic Terminology
Trait - any characteristic that
can be passed from parent to
offspring
Heredity - passing of traits from
parent to offspring
Genetics - study of heredity
More Terminology
Genotype - gene combination
for a trait (e.g. RR, Rr, rr)
Phenotype - the physical
feature resulting from a
genotype (e.g. red, white)
 Alleles - two forms of a gene
(dominant & recessive)
 Dominant - stronger of two genes
expressed in the hybrid;
represented by a capital letter (R)
 Recessive - gene that shows up
less often in a cross; represented
by a lowercase letter (r)
Genotypes
 Homozygous genotype - gene
combination involving 2
dominant or 2 recessive genes
(e.g. RR or rr); also called pure
 Heterozygous genotype - gene
combination of one dominant &
one recessive allele (e.g. Rr);
also called hybrid
Types of Genetic Crosses
Monohybrid cross - cross
involving a single trait
e.g. flower color
Dihybrid cross - cross involving
two traits
e.g. flower color & plant height
Punnett Square
Used to help
solve genetics
problems
Genotype & Phenotype in Flowers
Genotype of alleles:
R = red flower
r = yellow flower
All genes occur in pairs, so 2
alleles affect a characteristic
Possible combinations are:
Genotypes
RR
Rr
Phenotypes
RED
RED
rr
YELLOW
Generation “Gap”
 Parental
P1 Generation = the parental
generation in a breeding experiment.
 F1 generation = the first-generation
offspring in a breeding experiment. (1st
filial generation)
 From breeding individuals from the P1
generation
 F2 generation = the second-generation
offspring in a breeding experiment.
(2nd filial generation)
 From breeding individuals from the F1
generation
Following the Generations
Cross 2
Pure
Plants
TT x tt
Results
in all
Hybrids
Tt
Cross 2 Hybrids
get
3 Tall & 1 Short
TT, Tt, tt
 Trait:
P1 Monohybrid Cross
Seed Shape
 Alleles: R – Round r – Wrinkled
 Cross: Round seeds x Wrinkled seeds
RR
x
rr
r
r
R
Rr
Rr
R
Rr
Rr
Genotype: Rr
Phenotype: Round
Genotypic
Ratio: All alike
Phenotypic
Ratio: All alike
P1 Monohybrid Cross Review
 Homozygous dominant x
Homozygous recessive
 Offspring all Heterozygous
(hybrids)
 Offspring called F1 generation
 Genotypic & Phenotypic ratio is
ALL ALIKE
 Trait:
F1 Monohybrid Cross
Seed Shape
 Alleles: R – Round r – Wrinkled
 Cross: Round seeds x Round seeds
Rr
x
Rr
R
r
R
RR
Rr
r
Rr
rr
Genotype: RR, Rr, rr
Phenotype: Round &
wrinkled
G.Ratio: 1:2:1
P.Ratio: 3:1
F1 Monohybrid Cross Review
 Heterozygous x heterozygous
 Offspring:
25% Homozygous dominant RR
50% Heterozygous Rr
25% Homozygous Recessive rr
 Offspring called F2 generation
 Genotypic ratio is 1:2:1
 Phenotypic Ratio is 3:1
…And Now the Test Cross
Mendel
then crossed a pure & a
hybrid from his F2 generation
This is known as an F2 or test cross
There are two possible test
crosses:
Homozygous dominant x Hybrid
Homozygous recessive x Hybrid
F2 Monohybrid Cross (1st)
 Trait:
Seed Shape
 Alleles: R – Round r – Wrinkled
 Cross: Round seeds x Round seeds
RR
x
Rr
R
r
R
RR
Rr
R
RR
Rr
Genotype: RR, Rr
Phenotype: Round
Genotypic
Ratio: 1:1
Phenotypic
Ratio: All alike
F2 Monohybrid Cross (2nd)
 Trait:
Seed Shape
 Alleles: R – Round r – Wrinkled
 Cross: Wrinkled seeds x Round seeds
rr x Rr
R
r
r
Rr
Rr
r
rr
rr
Genotype: Rr, rr
Phenotype: Round &
Wrinkled
G. Ratio: 1:1
P.Ratio: 1:1
F2 Monohybrid Cross Review
 Homozygous x
heterozygous(hybrid)
 Offspring:
50% Homozygous RR or rr
50% Heterozygous Rr
 Phenotypic Ratio is 1:1
 Called Test Cross because the
offspring have SAME genotype as
parents
Review of Monohybrid Crosses
Inheritable
factors or genes are
responsible for all heritable
characteristics
Phenotype is based on Genotype
Each trait is based on two genes,
one from the mother and the other
from the father
Mendel’s Law of Independent
Assortment

Allele
pairs separate
independently during
formation of gametes
(meiosis)
Dihybrid Cross
Traits:
Seed shape & Seed color
Alleles: R round
r wrinkled
Y yellow
y green
RrYy
RY Ry rY ry
x
RrYy
RY Ry rY ry
All possible gamete combinations
Dihybrid Cross
RY
RY
Ry
rY
ry
Ry
rY
ry
Dihybrid Cross
RY
Ry
rY
RY RRYY
RRYy
RrYY
RrYy
Ry RRYy
RRyy
RrYy
Rryy
rY RrYY
ry RrYy
RrYy
Rryy
rrYY
rrYy
ry
rrYy
rryy
Round/Yellow:
Round/green:
9
3
wrinkled/Yellow: 3
wrinkled/green:
1
9:3:3:1 phenotypic
ratio
Dihybrid Cross
Round/Yellow: 9
Round/green:
3
wrinkled/Yellow: 3
wrinkled/green: 1
9:3:3:1
Test Cross
A
mating between an individual of
unknown genotype and a homozygous
recessive individual.
 Example:
bbC__ x bbcc
BB = brown eyes
Bb = brown eyes
bb = blue eyes
CC = curly hair
Cc = curly hair
cc = straight hair
bC
bc
b___
Test Cross
Possible
bc
results:
bC
b___
C
bbCc
bbCc
or
bc
bC
b___
c
bbCc
bbcc
Genetic Practice
Problems
Breed the P1 generation
tall
(TT) x dwarf (tt) pea plants
t
T
T
t
Solution:
tall (TT) vs. dwarf (tt) pea plants
t
t
T
Tt
Tt
produces the
F1 generation
T
Tt
Tt
All Tt = tall
(heterozygous tall)
Breed the F1 generation
tall
(Tt) vs. tall (Tt) pea plants
T
T
t
t
Solution:
tall (Tt) x tall (Tt) pea plants
T
t
T
TT
Tt
t
Tt
tt
produces the
F2 generation
1/4
1/2
1/4
1:2:1
3:1
(25%) = TT
(50%) = Tt
(25%) = tt
genotype
phenotype
Dihybrid Cross
A
breeding experiment that tracks
the inheritance of two traits.
Mendel’s “Law of Independent
Assortment”
Each
pair of alleles segregates
independently during gamete
formation
Formula: 2n (n = # of heterozygotes)
Question:
How many gametes will be produced for
the following allele arrangements?
Remember: 2n (n = # of heterozygotes)
1.
2.
3.
Rr Yy
Aa Bb CC Dd
Mm Nn Oo PP QQ Rr ss Tt Qq
Answer:
1. RrYy: 2n = 22 = 4 gametes
RY
Ry
rY ry
2. AaBbCCDd: 2n = 23 = 8 gametes
ABCD ABCd AbCD AbCd
aBCD aBCd abCD abCD
3. MmNnOoPPQQRrssTtQq:
2n = 26 = 64 gametes